This invention relates to external protection material for the outside skin structure of vehicles, e.g., missiles and space vehicles, which must resist multi-environmental conditions.
Missiles and space vehicles must have an external protection system which must endure not only normal handling procedures before launch but the launch environments as well as in-flight environments. The pre-launch environment is reasonably obvious. However, the launch environment includes the requirement to survive a launch immediately after a direct hit at the launch site while the immediate atmosphere is full of relatively large particles of debris, shock waves, and the x-rays produced by an atomic explosion which produce large impulsive mechanical loads. The in-flight environment, although it may include rain, dust, and micro meteorites, is generally a thermal problem including both heat generated by radiation and aerodynamic heating.
While all materials ablate when subjected to sufficient heat, what is required is a material that ablates at a rate that matches the heat input which is generated as a result of endo-atmospheric and exo-atmospheric radiation combined with the aerodynamic heating. The material system should have a relatively low thermal conductivity to avoid overheating the vehicle structure while having an ablative energy or heat of transformation to match the heat input. Also, the protective material which remains after partical ablation must protect the vehicle during its final environment. At the same time, the material system must be resilient and tough enough to absorb the impact of foreign objects, shock waves, X-ray impacts, etc. without undue penetration or tearing away.
Prior art history of these protective material systems includes U.S. Pat. No. 4,365,029, assigned to the same assignee as the present invention, which was a black copolymer made by combining an ethylene acrylic elastomer with polyvinyl chloride and carbon black to form an abrasion resistant coating. This material may be calendared into sheet stock or is used, i.e., as repair coating or as an adhesive. In sheet form this material is a "bond-on" and was actually the forerunner to Vamac 22B. Vamac 22B is a "bond-on" and is the external protection material used on a known missile `X`. It is also the baseline material used herein for comparative test analysis purposes. Vamac.sup.R is a registered trademark for an ethylene acrylic type elastomeric polymer manufactured by the DuPont Chemical Company. Vamac 22B is a modified rubber elastomeric polymer and will be described in detail infra.
U.S. Pat. No. 4,428,998 is a typical laminate of fabric plies coated with finely divided microspheres, all of which are fabricated as a laminate and bonded to the missile skin. This reference does teach an elastomeric material, but not with the present modifiers, uses fabric plies, and is a "bond-on."
U.S. patent application Ser. No. 581,186, now U.S. Pat. No. 5,196,228 and assigned to the same assignee as the instant case, teaches a polysiloxene elastomer with, among other modifiers, boron nitride which is also a modifier in the present invention.
Assignee's U.S. patent application Ser. No. 813,911, currently pending before the United States Patent Office, is another "bond-on" of a material system comprising a carbon based ethylene acrylic elastomer followed by an aramid cloth and finally a white boron nitride based elastomer as the outside coating.
Assignee's U.S. patent application Ser. No. 813,812, currently pending as a patent application, incorporated herein by reference, teaches a prepreg wherein a yarn is preimpregnated with an ethylene acrylic elastomer modified with Kevlar.sup.R pulp, a registered trademark of the DuPont Chemical Company, and boron nitride which is a material which was specifically invented to accommodate the subject invention.
One object of the present invention is to provide an integral overwrap of an elastomeric material system directly on the missile skin or substrate voiding the bonding on step required of a prefabricated and a precured material system.
It is a further object of the present invention to provide an integral overwrap protective material for a missile which is co-cured with the missile skin or substrate.
It is a further object of the present invention to provide an integral overwrap of the missile skin for a protective system which may be wet wrapped or dry wrapped using a prepreg which may be further protected by a final elastomeric coating.